Very old, already read threads pop up as unread

Every once in a while I witness a strange behavior. I read all my unread posts, that number decreases as expected. When I think I'm done I'm signalled of about 60 new threads with unread posts. It's exactly two pages in the list. After I manually opened every single one of them to mark them "read" again. A new set of exactly two pages with unread posts pops up. In the actual case, the posts last back to September 23rd to 26th. I only get rid of the stale posts by pressing the "mark all as read" button.